In ink-jet printing methods, droplets of ink are projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to obtain printed matters on which characters, images, etc., are printed. The printed matters are required to have high optical density and gloss, whereas the inks for ink-jet printing are required to have a high ejection property. In addition, color filters for liquid crystal displays are required to exhibit high contrast ratio and brightness. Therefore, pigments used in the inks for ink-jet printing and color filters are in the form of fine pigment particles having a very small primary particle size.
As the method of obtaining a fine organic pigment by reducing a primary particle size of an organic pigment as a raw material, there are extensively used wet kneading and milling methods such as solvent salt milling, dry milling methods, etc. The solvent salt milling means a milling method of atomizing a powder, etc., by a wet kneading method using a water-soluble inorganic salt as pulverization media. For example, by mechanically kneading the raw material organic pigment and the water-soluble inorganic salt together with a water-soluble organic solvent, it is possible to pulverize the pigment and reduce a primary particle size of the pigment.
Patent Literature 1 discloses a process for producing a fine organic pigment, including the step of kneading a mixture of a raw material organic pigment, a milling aid and a water-soluble organic solvent by a solvent salt milling method, in which the kneading is conducted in the presence of an organic substance containing a specific organic acid metal salt.
Patent Literature 2 discloses a process for producing a pigment composition having excellent gloss and stability, in which a crude phthalocyanine pigment is subjected to a wet milling treatment in the presence of a phosphoric acid ester or a sulfuric acid ester of a polyethylene glycol monoether.